1. Field of the Invention
This invention pertains to methods and apparatus for sorting homogeneous products flowing in a product stream based on color reflectivity of the products in one or more reflectivity bands of light and more specifically for consistently color sorting such products using a dark contrasting background and pixel detectors associated with a viewing frame.
2. Description of the Prior Art
Homogeneous products, such as coffee beans, are sorted or graded most conveniently in optical or color sorting machines based on the amount of light that is reflected in one or more bands of reflectivity. Such sorting is sometimes referred to as being either monochromatic or multichromatic. For example, darker beans reflect less light in certain selectable frequency bands than lighter color beans. When unsorted or ungraded beans are conveyed, typically by gravity feed down a chute, they are transported through an optical viewing station and observed by one or more optical sorting assemblies. Each assembly typically includes one or more lights directed at the stream of products to be sorted, which cause reflection from the products in one or more frequency bands of light selected for use for sorting purposes. A photodetector sensitive to a sorting band is positioned to receive the reflected light and produces an electrical signal that is proportional to the amount of reflectivity in the sorting band. If the amount of reflected light is within acceptable limits, then the amplitude of the signal produced is within acceptable limits. However, when a product is photodetected that is too dark or too light, the reflectivity in the color sorting band will likewise respectively be less than or more than the respective predetermined threshold levels or limits and, thus, will produce an electrical signal having an amplitude that is either too small or too large. These electrical limits are sometimes referred to as "trip line values".
When a trip line value is exceeded in either direction from the acceptable amplitude range, an ejection signal is produced at a time when the detected product to be removed from the product stream is opposite an ejection mechanism, usually an activated jet of air.
It is very common to view the product stream simultaneously from more than one direction, each direction being associated with a similar optical sorting assembly so that a spot on a product that should cause its ejection is not hidden from view. That is, a spot will always be seen by at least one of the assemblies regardless of how the products are oriented as they tumble and rotate in the product stream trajectory. For example, it is common to employ three optical sorting assemblies separated at 120.degree. locations in a plane orthogonal to the product stream trajectory. An ejection signal from any one of the three assemblies causes a product ejection.
In addition, it is also common to employ additional optical sorting assemblies with photodetectors sensitive to a second frequency color band, which would classify such sorting as biochromatic. Thus, acceptable products must be within an acceptable reflectivity range in each of two color sorting bands, not just one. In other words, a product that has a reflectivity response outside of the acceptable range in either color sorting band will be ejected from the stream.
Since reflectivities in selected color bands are the criteria for sorting, it is desirable to detect only the product and not background. In fact, if the background is not a contrasting color from the products being sorted, there may be a difficulty in detecting when a product enters the viewing station. Thus, a contrasting background achieved by painting or by lighting or a combination of both is useful. It is obvious that the background alone appears between products in the product stream and can be ignored or largely eliminated from the product detection operation by only activating the photodetection operation on the basis of detecting the presence of a product in position before the photodetectors at the time of generating the photodetection responses. The contrast background, which can be dark or black, allows the detection of the onset of a product.
However, it is not possible to totally eliminate the effect of background only by timing the photodetection event since the photodetectors observe everything within their respective fields of view whenever they are activated. This observation may simultaneously include a product and a portion of the background, as well. A procedure employed to minimize the amount of background simultaneously observed with the product is the use of a viewing window or frame, usually an elongated slot, placed in front of the photodetector. Such a frame cuts off viewing outside of a small exposed viewing area, where the products are expected to pass. The smaller the frame, the greater the elimination of extraneous reflectivity from the background.
Frames that are too small, however, cause other problems. One problem is that by reducing the size of the viewing area, there is also a reduction in the quantity of products that can be sorted in a given amount of time. If the viewing area is reduced too much, then some products may not be completely viewed and very small products possibly could be missed altogether. A frame that is smaller than the diameter or width of many of the products in the stream also may block off or shield a spot on a product that should cause its ejection or removal as being non-acceptable. Therefore, viewing frames are typically sized so that a portion of the background is viewed along with the detected products.
Although it is possible to appropriately contrast the background with the acceptable sorting color bands, it is not easy to maintain a background color when achieved by painting or lighting because of dust and other contaminants that accumulate and discolor the background. Since the background is photodetected to whatever extent it fills the frame that is not covered by a product, a constantly changing background can have an effect on sorting sensitivity. Moreover, it should be noted that a contrasting background used in conventional sorting machines has a greater effect on a small product than a larger one. This is because a small product passing by the viewing frame may be only a small fraction of the overall frame and, therefore, its effect is not as large as the effect of a proportionally larger product with respect to the background reflectivity during sorting. Thus, it may be possible for small products that should be ejected to escape being ejected. By contrast, a large product may block out most of the background and will be the major influence on the overall reflectivity for the observation of that particular product. Thus, the background is not a consistent influence on each product observation, even though sorting by color is supposed to be size insensitive.
There are other sorting machines that do not employ viewing frames and which do use matching backgrounds. Not only do such machines suffer some of the same problems as discussed above with respect to maintaining a background color in operation, it is also apparent that a machine employing a product matching background is not easily converted to sort a different product since the background would also have to be changed.
It is desirable to sort products on the basis of product reflectivity without taking background into account, on the basis of reflectivity independent of size or a particular trajectory and to be able to use the machine for sorting different products without changing background.
A system for calculating the amount of frame fill in a sorting machine is disclosed in U.S. Pat. No. 4,647,211, commonly assigned ("'211 patent"). In such a system, a light emitting diode (LED) in the infrared spectrum is employed as the light source to illuminate the area behind the product stream. A light box using a diffuser is employed, the backgrounds thereof being established to contrast with the product being sorted. The light from the LED is modulated by pulsing the LED at a frequency above the response frequency or frequencies used for sorting the products as reflected from the light of the incandescent lamps generally illuminating the products in the product stream. For systems employing more than one optical assembly in the same orthogonal plane, the respective LED light sources of the assemblies are also time multiplexed so that only one light source is operable at a given time. A photocell is located on the opposite side of the product stream trajectory from the LED with which it operates and a viewing frame is established in front of the photocell. Thus, a product in the product stream obscures the modulated light beam observed by the photocell in that part of the frame occupied by the product. The amount of pulsed voltage output from the photocell to its preamplifier is proportional to the amount of pulsed light resulting from the non-obscured portion of the frame. Thus, the resulting signal distinguishing product from background is useful in allowing the reflectivity monitoring to occur only with respect to product and not with respect to background.
It is apparent that the scheme outlined above and disclosed fully in the '211 patent for determining frame fill involves modulation and time control electronics, as well as a special frequency LED light source that is apart from the ambient incandescent light sources. Although such scheme does achieve the desired results, it is complex when compared to the instant arrangement not employing modulation or time multiplexing.
Therefore, it is a feature of the present invention to provide an improved method of sorting by detecting the background apart from the products in a product stream by using a contrasting background, preferably a black cavity.
It is another feature of the present invention to provide an improved method of sorting by determining the percentage of overall frame fill that is occupied by the product being viewed through the viewing frame compared with the overall width of the frame and using that information to correct the amplitude of the reflectivity signal so that all products of the same color in the product stream produce the same corrected amplitude value.
It is still another feature of the present invention to provide an improved apparatus for implementing the inventive methods disclosed herein.